According to the information given in the exercise, the lines A and B are parallel.
When a Transversal intersects those parallel lines, several pair of angles are formed. One of these pairs of angles are the "Interior angles on the same side of the transversal". These angles are supplementary, which means that their sum is 180 degrees.
In this case you can identify that the angle that measures 125 degrees and the angle 6 are Interior angles on the same side of the transversal. Therefore, you can set up the following equation:
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